The present invention is directed to an arrangement for coupling optional auxiliary devices to terminal equipment of private branch exchanges, particularly via an auxiliary device interface defined according to ETSI (European Telecommunications Standards Institute).
The auxiliary device interface ("AEI"; "Additional Equipment Interface") defined according to ETSI is increasingly gaining in significance with the area-covering introduction of Euro-ISDN (DSS1). Such an interface is defined in "ETSI DRAFT pr I-ERTS 300 245-4", Integrated Service Digital Network (ISDN), Technical characteristic of telephony terminals, Part 4: Additional Equipment Interface (AEI), June 1993. The AEI specification according to ETSI defines a combination of four interfaces, whereby the terminal equipment can support one interface or a combination of interfaces. These interfaces are: an analog 4-wire interface (X); a digital 4-wire signalling interface (Y); an analog 2-wire interface (Z.sub.A); an ON/OFF or: IN/OUT! signal interface (Z.sub.B)
This interface defined according to ETSI is an additional equipment interface that sequences the communication between Euro-ISDN terminal equipment and additional equipment (such as, for example, a name key console or a selection aid interface for a personal computer) with functional messages. Euro-ISDN terminal equipment are based on the protocols Q.921 and Q.931/932 according to the CCITT Standard. The exchange and the terminal equipment are thereby automatic status devices.
The functional AEI unit is locally arranged in the terminals equipment and requires a local DSS1 and AEI automatic status device. These local demands represent a considerable added outlay for a Euro-ISDN terminal equipment when activating the AEI unit. Due to the function distribution, each performance feature of the AEI is completely and mandatorily defined in the signalling. A modification or expansion of the performance features in the AEI therefore requires a software modification in the Euro-ISDN terminal equipment.
It can also be necessary to activate an AEI unit for system terminal equipment such as, for example, of the type Symphony of Siemens that have a stimulus function division and that do not employ DSS1 as a signalling protocol. In stimulus terminal equipment, the function distribution is such that only the exchange is an automatic status device, whereas the terminal equipment is a pure input/output device. This requires a smaller processor and less program memory space. Such proprietary stimulus terminal equipment are therefore the only present possibility for manufacturing what are referred to as absolute low-cost terminal equipment, since the functional Euro-ISDN terminal equipment require component parts and software that do not allow the prices of the stimulus terminal equipment to be achieved. However, the local utilization of an AEI unit also leads to a considerable outlay in the terminal equipment given stimulus terminal equipment and, correspondingly, is economically and technologically unfeasible.